The chromium coatings are extensively used for decorative applications or for the protection of parts against wear and corrosion. Hard chromium seems irreplaceable as a metallurgic coating in the field of surface treatments, taking into account its physico-chemical characteristics: good wear resistance due to a low friction coefficient, high chemical strength, hardness, and advantageous aesthetic finish. The coatings for the two above-mentioned applications differ only by their thickness (small for decorative finishes, large for protection). Chromium plating thus is called for as a treatment of choice in numerous industrial sectors (automobile, furniture, medical instruments, and optics).
The metal coatings based on chromium or other metals are essentially obtained by the electroplating bath method that makes possible the treatment in a stream of small parts like flat production of larger dimensions. Chromium plating is a process that is simple and easy to use, having low operating costs and making it possible to treat parts at a very low temperature (less than 100° C.). However, the thus obtained chromium coatings are often amorphous, having low hardness values. Annealing at 500° C. is then necessary to reach hardness values on the order of 1000 HV, whereby this value tends to drop at a higher temperature (300 HV at 700° C.). Furthermore, the films are tension-constrained, which may bring about poor adhesion with the necessity for an intermediate layer (Ni). Finally, the films that are obtained are microcracked, which weakens them with regard to corrosion, in particular if the coating protects a part that is sensitive to localized corrosion, made of stainless steel, for example. In addition, this wet surface treatment requires the use of chromium solution baths that are most often hexavalent, or sometimes trivalent, although the quality of the films that are obtained is generally not as good. Whereby the hexavalent chromium is known for its carcinogenic effects, the manufacturers that use these bath chromium-plating processes are working toward the constraining objective of “zero wastage.” Incidentally, European environmental standards will prohibit its use starting in 2007.
Alternative, so-called suitable, techniques for dry deposition have been proposed to obtain chromium coatings with properties that are similar to those of electrolytic hard chromium, among which are found the chemical deposition techniques. For example, the chemical vapor-phase deposition (CVD, for Chemical Vapor Deposition) of a metal from a cement that consists of a metal powder in contact with a high-temperature volatile reducing compound is known. Regarding the chromium-based depositions, the metal powder is confined in the presence of a halide (NH4F or HF) and brought to high temperature (950-1050° C.). By chemical displacement reaction, a volatile chromium halide forms and is reduced in the presence of the formed dihydrogen. This process can operate at atmospheric pressure, but the depositions are obtained only at high temperature because of the halide-type metal source that is used. The so-called conventional CVD processes, directly using corresponding halide vapors as a chromium source, operate under dynamic vacuum and at high temperature.
These two CVD processes, strongly activated thermally, make possible the treatment of complex surfaces, but their primary drawbacks are (i) the use of halide precursors that are thermally robust, toxic, corrosive, and with limited volatility, and (ii) with high deposition temperatures that, on metal substrates, in particular steel or technical alloys that have specific properties, will cause dimensional deformations, microstructure modifications, changes in crystalline structure, leading to the degradation, and even the loss, of the specific functional properties of the treated part.
To bring the deposition temperatures to a value that is less than 550° C., considered to be the critical holding temperature of steels and alloys, organometallic molecular precursors have been used in the laboratory (MOCVD process, for Metal Organic CVD). Taking into account the low volatility and the thermal instability of these organometallic precursors that often come in powder form, it is necessary to operate under reduced pressure. In addition, the extended heating of the precursor in the sublimation zone, even at low temperature, can degrade the reagent before it arrives in vapor form at the deposition zone, thus involving problems of reproducibility in terms of flow rate, the initial reactive gas composition, and therefore the deposit quality.
The precursors that are used are preferably selected from among sandwich compounds in which the metal atom of degree of oxidation zero is linked to two aromatic cycles that are optionally substituted by alkyl groups. Obtaining a metal deposit then comes up against the fact that during the decomposition of the precursor that proceeds from the separation of the metal-ligand bonds, the hydrocarbon ligands also undergo decomposition and add their carbons, which causes the formation of ceramics such as chromium carbide and not metallic chromium. To eliminate this drawback, a chlorinated or thionic additive is introduced into the reactor: the additive in powder form is introduced in vapor state via a suitable peripheral line, by entrainment in a gas stream, like the precursor. It poisons the reactive surface and prevents the total decomposition of the ligands. The low volatility of the powders that are used as additives is an additional reason to operate under reduced pressure. The introduction of two reagents in powder form by independent lines, in addition to making the system complex and the increased investment cost that it involves, also poses different problems of adjustments, stability of the reactive gas mixture, and therefore reproducibility.
Physical vapor-phase deposition (PVD) processes have also been proposed as alternatives to wet processes. Metal chromium deposits have been made from chromium targets. In addition, solid chromium solutions are made by implanting nitrogen or carbon, having hardness values that can range up to 25 GPa, compared to pure chromium that is obtained without ionic implantation (6 GPa). However, the forced vacuum that is necessary to these processes, the high cost of equipment, and the low deposition rates constitute serious limitations to an implementation for a continuous surface treatment.
It thus appears that it has not been possible until now to make deposits of hard metal coatings at low temperature and under atmospheric pressure, applicable for the protective or decorative treatment of metal parts, in particular the parts with a large surface area, without recourse to bath treatments whose drawbacks have been mentioned above. This invention has as its object to meet this requirement. It has as its object a process that combines the technique of the chemical vapor-phase deposition and the direction liquid injection, the so-called DLI-CVD, a process according to which a solution that comprises an organometallic precursor of the metal compound to be deposited and a chlorinated additive is introduced into a reactor.
The principle of the DLI-CVD technique is to introduce into a chemical vapor-phase deposition chamber a solution of a precursor of the element to be deposited by periodic injection of droplets of said solution that are entrained by a vector gas to the deposition chamber. A DLI-CVD device has been developed for the deposition of thin layers of oxides on the microelectronics plates, but it has never been used for the deposition of non-noble metals at atmospheric pressure. This is due to the fact that work at atmospheric pressure, which offers a large advantage for the industrial production at low cost for large parts, imposes particular conditions that have not been defined to date. Furthermore, the operating conditions are not easily transposable to the deposition of metal layers. Actually, the works relative to obtaining metal layers by DLI-CVD involve only noble metals (Ru, Ag, . . . ), i.e., metals that have little or no affinity for the light elements C, O and N that are present in the commonly used solvents. In addition, in the case of microelectronic layers, carbon contamination is to be avoided. However, whereby the latter results from the large quantity of hydrocarbon solvent in the reactor, it is more easily controlled when the decomposition is conducted in the presence of oxygen or an oxidizing gas (N2O, H2O, . . . ). For coatings that are designed for microelectronics, oxidized solvents are often used, moreover. A large part of the carbon is then eliminated by combustion whereas an oxide film is deposited, which is not compatible with the purpose of this invention.